refactor(metrics): split report assembly into phases

[sturlese]

Summary

Refactor metrics.build_report() into explicit private computation phases while preserving its public contract and observable behavior.

Changes

• extract full-history weekly aggregation and adoption inputs
• isolate decision-window workflow KPI assembly
• separate governance completion from report total finalization
• add characterization tests for temporal boundaries, all-history adoption, orphan evidence, weekly aggregation, and stable ordering

Testing

• python -m pytest --cov=flightdeck --cov-fail-under=85 — 258 passed, 94.48% coverage
• python -m pytest tests/test_metrics.py — 29 passed before and after the production refactor
• python -m pytest tests/test_cli_and_html.py — 33 passed
• ruff check src tests — passed
• offline demo, ledger verification, and dashboard generation — passed

Notes

• no public API, dataclass, formula, serialized output, or persisted format changes
• existing future-run inclusion and all-history adoption semantics are intentionally preserved and characterized
• Build Squad tester and architecture auditor verdicts: PASS
• no documentation change is required because the module boundary remains evidence-to-KPIs